There isn't any update speed option on the default MS RSS Feed gadget. Mine seems to be working though, looks like it updates every 20-30 seconds or so.
Try these steps in an elevated command prompt.

Information
1. Check if Task Scheduler is running
sc queryex schedule
Look at the "STATE:" line, it should read "4 RUNNING" if it's not running, then start it
sc start schedule
2. Check if RSS Download Engine is running
reg query HKCU\Software\Microsoft\Feeds /v SyncStatus
Look at the "Status" line, it should read "0x01" or "0x1". If it isn't enabled, then enable it
msfeedssync enable
3. Check if task is corrupted
schtasks /query | findstr /i "user_feed"
Look for
ERROR: The task image is corrupt or has been tampered with.
ERROR: Task cannot be loaded: User_Feed_Synchronization-{..
If corrupted, then delete and re-create the task
msfeedssync disable
msfeedssync enable
Source: Feeds not updating?
Mine looks all good, so no need for me to run the last commands.