If your sailing true speed conditions IE 130-140 degrees, then your lines are best when set way back. Your back hand will almost be touching back line, and your front hand is no were near the front line. This is the correct way as you use your body weight to sheet instead of pulling with the back hand and inducing a spin out. Coming back up wind should be a real struggle and exhausting, then your are going to be nicely set up for a proper down wind run
Forearm pump is common among speedies. Here's a few things you can do that might help, but Im sure you will be aware of these basic things already.
1: Find a stance and line length/position that allows to feel like you can let go of rig with your front hand (but dont unless your brave/stupid). This may take 10 or more runs of adjusting/altering trim to find what works "that day". Every session will be slightly different.
2: Start applying a couple of extra cm to your downhaul to give lighter feeling to your sail. Most speedies go way past normal recommended settings.
3: Make sure you have had plenty of carbs the night before (and no beer, sorry). Top up on porridge in the morning. Eat bananas (for potassium) on the day to help flush out lactic acid from your muscles. Stay well hydrated. Come back in for a break before your tank runs on empty for a 20 minute break and do a few simple forearm stretches and give your forearms a quick basic deep message to get blood flowing before going back out.
4: Make sure your wetsuit still has plenty of flexiblity in the forearm area. Wetsuits loose there flexibilty after time, and can cause forearm tiredness.
5: If you have your lines close together, open them up (a lot) aka SU11. More and more speedies are using their lines further apart.
6: If not already, start working pumping some iron to regain your arm strength (and some more).
7: Last but not least, If your using an older thicker boom, try and have a go of a narrower diameter boom. Chinook, NP and simmer do good 29mm booms. A thicker diameter will give you tired forearms quicker. I recently got an AL360 E3 slalom and the v-grip combined with the new school shape does help me to keep going longer.
Are you posting on gp3s.com?
Non of this might work for you as there is more than one way to cook a goose, but good luck and good speeds