Of course Salvador Perez wouldn't do very well on ground balls. He's a catcher. Catchers are slow. The average production on grounders this year is .235 BA / .254 SLG. Catchers hit more like .216 BA / .235 SLG. Duh. However, Perez is on a different level. A lower level. Unfortunately, he's been descending to this level for some time.
PRD calculated by 1.7*BA+SLG. A quick-and-dirty OPS-like measure that more appropriately weights getting on base.
This year, so far, Perez is hitting .194 BA / .213 SLG on grounders. What used to be a strength in 2011-2013 was something resembling a problem in 2014 and is now something that should be examined. In other batted ball areas, Perez is doing OK; he's crushing fly balls and still doing OK (though not great) on line drives. So, why the trend?
There are two potential areas for investigation - the shift and actual quality of contact. Unfortunately, shift information is not made public and quality of contact information is not as granular as I would like right now. I'll do what I can.
First, Perez doesn't seem particularly vulnerable to the shift. He's actually increased his rate of opposite-field ground balls for three years in a row, and he's decreased his pull rate of grounders two years in a row. This year, he hits roughly three pulled grounders for every opposite field grounder. That's not in shiftable territory. I can't say definitively whether the shift has hurt him, but I can say that his current pull tendencies don't necessitate a shift in the first place. Interestingly, there is that correlation between increased opposite-field rate and declining production, but I don't know if that's just a correlation or an actual cause.
Second, the quality of contact stuff. StatCast is supposed to help bring this data into the public domain more, but I'm still iffy on whether or not to trust it. For what it's worth, Perez ranks 110th out of 436 in average ground ball velocity among players with at least 20 ABs of batted ball velocity data. There's some survivor bias there (better players will get more ABs and presumably have higher velocities than those players who don't get many ABs). So Perez's average batted ball velocity on grounders isn't terrible, but it's not great.
FanGraphs also has some quality of contact information. Their information says that this year Perez is hitting the lowest rate of hard-hit grounders of his career. The rate of softly-hit grounders is the second-highest of his career. There are a bunch of medium-hit grounders, which aren't going to do a plodder like Perez any favors.
As far as this year goes, Perez's production lines up with what FanGraphs has on his soft / medium / hard rates of contact on grounders. If I had to guess, I'd say Perez just isn't hitting his grounders as hard as he used to. In addition, he's just rolling over on pitches. FanGraphs also has heat maps, so I looked at Perez's rate of ground balls in different parts of the strike zone.
It's tough to put together a trend over time with that kind of data, but I'll try. Back in 2013, Perez hit a lot more grounders. Specifically, he hit more grounders on inside pitches. 2013 was also Perez's last year of above-average grounder production (for a catcher). In 2014, Perez didn't hit as many grounders on inside pitches. Finally, in 2015, Perez's grounders come almost exclusively on pitches low and away or up. That helps explain why more of his grounders are going to the opposite field, and it might also help explain why his grounders aren't being hit as hard anymore.
It's possible that Perez has simply traded grounders for other stuff in those inside zones; since his fly ball production is up this year, maybe Perez has decided to elevate that inside pitch. His pull rate on flies is basically the same in 2014 and 2015, when the rate of grounders on inside pitches is lower. That pull rate on flies is lower in 2013, when he hit more grounders on those inside pitches. However, Perez's rate of soft-hit fly balls is down this year. It's hard to make anything definitive out of this due to limited sample sizes, but there might be something there.
1) Perez's production on grounders is falling
2) Perez is pulling grounders less
2a) Perez is not likely hurt by the shift
3) Perez doesn't hit grounders very hard
4) Perez is hitting more of his grounders on outside pitches than inside pitches
4a) This may benefit his fly ball production
Honestly, if there is a relationship here between his declining grounder production and better fly ball production due to elevating inside pitches more, I'll take the fly balls over the grounders.
If only he could fix that walk rate.