Solid polymer electrolytes (SPEs), though widely regarded as materials that can enable next-generation lithium metal battery with improved safety, extended stability, and high capacity, suffer from problems of low ionic conductivity. Myriads of strategies have been proposed to improve the performance of polymer electrolytes, but with little success, and the state-of-the-art polymer electrolytes are still based on LiTFSI dissolved in PEO, which have been proposed for more than 40 years. New design concepts are indispensable to improving the performance of SPEs. Using molecular dynamics simulation, we examine SPEs under nanoscale confinement, which has been demonstrated to accelerate the diffusion of neutral molecules such as water. While ion diffusion indeed shows an acceleration by more than two orders when the channel diameter decreases from 15~nm to 2~nm, the ionic conductivity does not show a paralleling increase. Instead, as the nanochannel diameter decreases, ionic conductivity shows a non-monotonic variation, with an optimal value above yet on the same order as its bulk counterparts. The reason for this trend is due to enhanced ion association with decreasing channel size, which leaves a smaller amount of effective charge carriers. This effect competes with accelerated ion diffusion, leading to the non-monotonicity in ion conductivity. We further show that this trend also appears in two-dimensional nanoslit pores. These findings not only manifest the intricacies of ionic transport behavior but also provide new concepts and necessary implications for designing composite SPEs.