bikey | Low memory footprint Map and Set implementation | Map library

I would guess the list would take about 44 bytes per object, or ~130Mb for 3m objects. This is a bit on the large side, but not incredibly so.

Some suggestions:

The list is well over the 87k limit for the small object heap (SOH), so it will be allocated on the large object heap (LOH). This is only collected in gen 2 and gen 2 collections can be expensive. To avoid this it is recommended to avoid de-allocation of gen2 objects as much as possible, i.e. allocate them once and then reuse them as much as possible.

You could fetch the list from the database in smaller chunks and update the list in place. Make sure each chunk is within the limit of the SOH. You might consider either locking the list to ensure it is not accessed while updating, or keep two alternating lists where you update one and then switch the 'active' list.

You are using a class for the TagEngineCacheResponse, this will cause a great deal of objects to be allocated. While these are small enough to fit on the SOH, they may, if you are unlucky, survive long enough to be put on gen 2 heap. While GC time is not greatly affected by un-referenced objects, it might still be better to use a value type and avoid the problem. Profile to make sure it actually helps.