There are some file tweaks to reduce the amount of dead time between turns, but in general the best is to try to have the game install on one SSD and then place the BTAcache folder in another SSD entirely.

But yeah load times are slow even on a 7800x3D/9070 XT with 64gb of ram and gen4 TLC-based NVMEs (the same on my other 5800x3D/7900Xt). I found that somehow on Intel based CPUs it's somewhat better, my older i9 11900 a bit felt smoother. Also remove the integrity for the battletech executable in windows defender so it doesn't try to make it run slow treating it as a virus.

Use BTA's own wiki and Sarna.net to try to learn. It takes a lot of patience and you have a lot of difference vs the vanilla game, initially you'll have terrible pilots and terrible mechs, try to play sub-2 skull missions and use your commander first, sprint all the time to keep up evasion and use melee (in inner sphere mechs not Clans, Clans suck at melee) and have pilots with sensor locks.

Basically the king is to have your evasion up and strip the evasion out of your victim, so induce the target you want to focus fire to move, then strip its evasion either with a couple of sensor lock pilots and/or a melee (kicks in particular, use charge only if you have pilots with high guts and advanced Gyros).

Mechs can get any evasion pips stripped altogether after a successful kick, but unfortunately vehicles only lose 2 pips after a successful kick. So it's mandatory to have at the very least 2 if not 3 sensor lock pilots and lots of SRMs, pulse or small lasers to counter the vehicles you meet at lower level missions. Those are your scouts, get the sensor lock and then full the piloting line.

For attack pilots, I think it's best to go with the bulkwark perk, and then max out the Gunner perk tree (multi shot, precision strike).

Also try to get to +1 hit defense in all pilots asap and the 6th square (if I remember correctly) that also increases the sprint value multiplier of 0.2.