NOTE: This issue is now obsolete, since Radix DLT moved from archive nodes to network gateways. If your wallet is slow, please check this updated thread: Why is the Radix Wallet and Radix Explorer slow?
The Radix desktop wallet needs to communicate with an Archive Node to retrieve data from the Radix ledger such as your XRD balance, or to make transactions. The Radix desktop wallet normally defaults to the official Radix archive node address and at times of peak demand, this can result in slow performance or in rare cases, inability to access the wallet.
Many members of the community run archive nodes in addition to those of the Radix team, and these alternative archive nodes can be selected within the wallet settings (see image below for some example community archive nodes). This often resolves any issues with poor wallet performance.