I think he's holding back his pace on purpose because he has absolutely no interest in playing Test cricket. About a year or so ago, he pondered retirement and as soon as word broke out, and the criticism came in, he realised the backlash and notoriety was not worth it, especially coming back after a spot-fixing ban.
It makes absolutely no sense as to how he is able to bowl close to the 150s in LOI's but his pace significantly drops when playing Tests. The only reason I can make out is that he does this on purpose. He makes enough money playing County, PSL, BPL and a bunch of other leagues and from advertisements considering he's a popular face in Pakistan. Playing Test cricket isn't really his priority and therefore he doesn't exert himself. He doesn't even mind being dropped.
If it were up to him, he'd have announced retirement by now. He's lost the passion, he's lost the jazba for Test match cricket, as have a lot of other bowlers in the past.
Are you hearing his grunts ?
Speed guns tell you 131 - 136.
He went back to FC cricket to solve some issues namely :
1. Improve his physical fitness and match fitness thereby revive his fast medium pace
2. Fix his action and bowl closer to the stumps with improved follow through
3. Revive his late in swing
4. Revive his seam up delivery going with the angle towards slip
3 and 4 needed fixing his wrist & release.
I see zero improvement in each area. Even if he does well here, (I hope & pray that he does) he won't be bowling match turning spells in any format on wickets which support batsmen. Boult does that Amir doesn't.
In last three years He has impressed me in following matches
1. Couple of BPL matches
2. Asia Cup T20 match btw Pak & India at Dhaka
3. Pakistan warm up match vs Somerset
4. CT final
That's it.
Overall he has been Average in all formats.
The Amir I wanted to see showed up only in 1 match ie Pak vs India March 2016 T20 match at Dhaka and CT final (to a lower extent) that's it.
I have followed him closely and I do not buy that in LOI matches he bowls 140+ kph. Most of his seam up deliveries are in range of 135 to 140 kph. Very few above 140 kph and rare deliveries between 144 to 147 kph.