this doesn't really make things faster, but I tried

vars/addHelmRepo.groovy

@@ -1,6 +1,4 @@
-def call(String repoName) {
-    def repoUrl = env.NEXUS_URL
-
+def call(String repoName, String repoUrl) {
     container('helm') {
         sh "helm repo add ${repoName} ${repoUrl}"
         sh 'helm repo update'

vars/deployHelm.groovy

@@ -1,7 +1,4 @@
-def call(String releaseName, String chartRepo, String chartName, String version, String image, Map values = [:]) {
-    def kubeApi =  env.KUBERNETES_API
-    def kubeToken = env.KUBERNETES_TOKEN
-
+def call(String releaseName, String chartRepo, String chartName, String version, String kubeApi, String kubeToken, String image, Map values = [:]) {
     container('helm') {
         def helmCommand = "helm install ${releaseName} ${chartRepo}/${chartName} --version ${version} --kube-apiserver ${kubeApi} --kube-token ${kubeToken} --kube-insecure-skip-tls-verify --set image=${image}"
 

vars/dockerBuildAndPush.groovy

@@ -1,7 +1,4 @@
-def call(String imageName, String dockerfilePath, String buildContext, String nexusUser) {
-    def dockerRepo = env.NEXUS_DOCKER_URL
-    def nexusPass = env.NEXUS_DOCKER_PASS
-
+def call(String dockerRepo, String imageName, String dockerfilePath, String buildContext, String nexusUser, String nexusPass) {
     container('docker') {
         sh "dockerd --insecure-registry http://${dockerRepo} &"
         echo "Started Docker daemon"