Slow starters.

Hello Bakers!

I Started baking a while ago and got in contact with sourdough after following a course on sourdough baking.
After the baking course they gave me a sourdough starter and explained me how to keep it alive.

after learning and keeping it for a while I noticed that it started getting slower. I bought a new starter (from the same place) and noticed it was a lot quicker. I know starters are never the same but this starter came form the same tub my starter came from after following the course a few weeks earlier.

Now a few weeks have passed again and the new quicker starter has lost its touch. It is just as slow as the older one and seems to take ages to rise after feedings. 

I always keep my starter in the fridge (i use it once a week to bake bread) and feed my dough 50 grams whole wheat flower and 50 grams lukewarm - warm water. I remove some of the old starter and keep 100 grams. after feeding I wait until it starts to rise and then put it back in the fridge. 

The only thing I have noticed could be the problem is that my fridge is very cold (around 6-9 degrees celcius). 
Whenever I make breads and leave them in the fridge for 10-20 hours to rise I notice they just stop. If i put them outside they continue rising after a while and I can still make a decent loaf but never rises the same way as when I used the fresh starter from the baker.

Now What I do when I bake is I feed my starter, wait until it starts rising and becomes "active". then take some of the starter to make my bread. 

Would it also be possible to just take some of the starter directly out of the fridge, feed it some dough (take the dough and water feeding from the recipe I use for my bread so the percentages still add up in the end) and then make the dough the next day to leave it rise again for several hours?

And any ideas on how I can make my dough more active and keep it active without leaving it outside of the fridge and feed it too often (if at all possible)

- Rens