Let's say there's a sword with no charge time reduction at lv 1, charge time reduction low at lv 5, then charge time reduction at lv 10.
If i obtained a uv of charge time reduction medium at lv 1, will it stack up and increase to charge time reduction very high at lv 10?
Yep, the ctr med at lvl 10 is added to whatever ctr uv you have on it. All uv's are additive. You can put numerical values of 1 for low, 2 for med, and 3 for high. Thus, 2 low's is equal to a med.