To be honest, I use to have a big ego when it came to increasing speed. I use to think "Okay I played it great 5 times now I can move up the tempo" or I would say "Okay I've played it for a minute I'm ready to move up the tempo." 99% of the time, I wasn't ready, even doing all of those things I wasn't ready.
The biggest thing about playing slow is muscle memory. And you cannot play fast without muscle memory. You can't. Plain and simple. The moment you are able to disengage the connection between "thinking about what you're playing" and "playing what you're playing", and you let it become automatic, then you are ready to build your speed by increasing the tempo.
If you cave in to the urge of increasing the tempo to judge your improvement, you are doing yourself a huge disservice. Speed should never be the goal. Muscle memory and autonomic playing should be the goal, because everything else will fall in line.
Otherwise, you will struggle with the sensation of always having to "fight" to keep the speed up. You don't want that. You'll hurt yourself. Great job for still working on it!