It runs slow on my computer as well (generally around 36 frames, and it's supposed to run in 50...)... I am giving speed high priority, but computers need to be a little stronger CPU-wise than my computer is, to manage to run it properly.
I'm actually working on tuning down Suntown's difficulty for the next release... like, finally removing the annoying left turret from the save point exit (almost impossible to not get hit by it). I've also been working a bit with the red and blue turrets (by the way, the soldier in the save point tells you that those turrets won't fire at you when you are Unarmed; is this obvious enough?), and the miniboss soldier you fight in a very cramped area will now have 10 Hp and be more of a hilarious Monty Python reference than an actual boss ("It's just a flesh wound!").
Question: what in Suntown do you find the most difficult to cope with? I can essentially only play that area with permanent Bullet Time thanks to my computer's CPU power, so I can't really hope to appreciate how difficult things are.
The two guardian bosses are a massive pain, especially since I have very limited ammo for my powerful weapons. The inability to buy any missiles or any serious firepower made that boss fight WAY too hard. I had to use my weaker weapons and exploit boss ai in order to get past it. Save for getting to know the absolutely ridiculous boss patterns and the fact that the second deals way too much damage far too freqeuntly is very annoying. Heck, if he had a charging phase before he used his lasers, he would have been more managable.
In his current state, (and this is from a while ago, so I'll be rewriting this if those two bosses change) the second guardian, in essence, beam spams you to death.
Ah. Well, in the most recent version of the game, I've removed the beam attacks and replaced it with the boss shooting missiles into the ceiling, causing blocks to fall down. And overall, the Guardian now relies more on spamming slow normal bullets. I've also redrawn its sprite a bit, so it looks different... I'm not completely satisfied with it yet, though.
Anyway, good suggestion to put ammo vending machines for the missiles, I've not updated them in forever so there's essentially only Machine gun, Hotrod, and Vector Blade ammo to buy. Do you think I need to add machines for both Thunderbolt and Wasp Swarm missiles, or only for one of them?
(Anyway, the improved Guardian will more or less be averagely easy to beat with machine gun and hotrod, since it has shields that blocks attacks from the sides.)
Oh, and if you're playing the old version (so old that the Guardian is still a rock head), the Sun Soldiers does also have a bad pattern that'll be fixed once I get 0.7.3 out and you download it. You'll have to erase your file and start over due to a save file incompitability, but the 0.7 version is worth it, and I daresay it will be months or years before the next file swapping time. Now all arrays are oversized so that I'll practically never run out of flags.
Update! Now version 0.7.4 is out. Features various improvements, such as the Deep Cave being possible to complete again (forgot to place a player object in one of the rooms, which made that room impossible to pass).
Also, now the Cooking System is up and running, with some 15 types of food being craftable.
Dialogue might need to be polished post-Deep Cave; I'm currently making changes in the storyline, making some lines make less sense than usually. However, that's only the case in one or two places, and so far into the game that you will probably not even get there before 0.7.5 ^_-.