Since update statistics uses dirty reads (transaction isolation level 0) for data-only locked tables, it can be run while other tasks are active on the server, and does not block access to tables and indexes. Updating statistics for leading columns in indexes requires only a leaf-level scan of the index, and does not require a sort, so updating statistics for these columns does not affect concurrent performance very much.
However, updating statistics for unindexed and non leading columns, which require a table scan, worktable, and sort can affect concurrent processing.
Sorts are CPU intensive. Use a serial sort, or a small number of worker processes if you want to minimize CPU utilization. Alternatively, you can use execution classes to set the priority for update statistics.
The cache space required for merging sort runs is taken from the data cache, and some procedure cache space is also required. Setting the number of sort buffers to a low value reduces the space used in the buffer cache.
If number of sort buffers is set to a large value, it takes more space from the data cache, and may also cause stored procedures to be flushed from the procedure cache, since procedure cache space is used while merging sorted values.
Creating the worktables for sorts also uses space in tempdb.