The fastest way I’m aware of to quickly go to paste-insert mode for a one-shot paste is tpope’s unimpaired, which features yo and yO, presumably mnemonics for “you open”. They’re only documented in his vimdoc, as:
A toggle has not been provided for 'paste' because the typical use case of wrapping of a solitary insertion is so wasteful: You toggle twice, but you only paste once (YOPO). Instead, press yo or yO to invoke o or O with 'paste' already set. Leaving insert mode sets 'nopaste' automatically.