This was very fun! I can totally see a game like this expanding on many fun mechanics especially as the sword might find ways to grow stronger. Maybe it can unlock basic elements like fire, electricity, stuff like that for a short time, like fire can be used to melt large ice blocks for example, maybe only if it can gain mana from a source and perhaps nearby runes and sigils can drain that mana (to add difficulty).
I didn't have any performance issues.
I could easily see something like, blocks that you maybe have to pierce into twice in order to break them to continue through the level. Especially in a game like this where you have to focus on being able to aim correctly. Maybe there are ways to force the runes and sigils at a radius to completely destroy them or nullify their effects for a short time, making you a little more powerful in specific areas of a level in order to do something a little more powerful (larger/onger jumps, breaking hard blocks, etc).
Maybe some areas have particularly more runes than others, making jumping even more difficult around that area. That could be followed up with finding a way to destroy some of them and returning to that part of the level to be able to beat it.
There is definitely a lot that you can do with this game! I'm excited to see it develop more!