Thanks for all the advice, I got a GBC but I have no experience in soldering or any hardware modding tbh so i cant do the Bass Mod and Noise Filtering Mod mod mentioned, it looks particularly complex. Is there anyone in the preferably in the UK who does these mods? 

I have been using bGB for my drafts but i feel the magic comes from the hardware so id rather stick with an actual gameboy (even if its a gbc) for my proper recordings. all tracks I've tried have had no slowdown even my jungle tracks which have liberal use of sample pitch bending which is cool but there is a noticeable high pitch noise

You can't.

For my own work I basically challenge myself to never write anything that bogs the CPU too much. Or if it does, just live with it. I figure If I want more 2X is the logical choice.

Have you tried putting in a tempo command? If you increase the tempo during the bits that are slowing down, you could cancel out the slowdown effect. (I've done this before, I...think?) Then put in a command to return the tempo to normal after those sections.

Also, don't use vibrato on more than one channel at once.

This doesn't help with things you've already written, but in the future you can halve the BPM and make the groove 3 ticks per step, I end up doing this for any song over 160 or any song I use a halfclock for. It rarely makes a difference unless you're one of those people who likes to use c commands for arps.

What BPM are you using? I find that anything above 180 will lag with heavy tables (most notably: V commands across multiple channels), so the only thing I can recommend is to cut your BPM in half and set your main groove to 3/3. Of course, you'll have to rework your tables and the R commands will get wonky. It might be worth a shot...

I get a lot of visual glitching but no noticeable slowdown.

i always found that fast tempos and freaky sample use caused slowdown as well

There really are very few times that I've been forced to use a CGB.

If you're not too concerned about authenticity, you could use BGB's audio record function. (But make sure you upgrade to the latest version first!) Then maybe mix it with a noise track from a real DMG for some realism, hehe.

slave your gb to a clock

Go with a GBC and never worry about it again! 

Optimize your tables. There is often more then one way to achieve the same sound.

GBC/GBA SP processors should handle pretty much anything you throw at them.

The GBC CPU can switch over to a faster setting.

If you are iffy about the GBC sound, you can perform the Bass Mod and Noise Filtering Mod, both of which are posted on this forum.

THe GBC would help, yes. You could also try splitting your sounds to different files and recording one at a time. For example, recording PU1, then PU2, then WAV, etc.